The role of puberty on physical and brain development: A longitudinal study in male Rhesus Macaques.

This study examined the role of male pubertal maturation on physical growth and development of neurocircuits that regulate stress, emotional and cognitive control using a translational nonhuman primate model. We collected longitudinal data from male macaques between pre- and peri-puberty, including measures of physical growth, pubertal maturation (testicular volume, blood testosterone -T- concentrations) and brain structural and resting-state functional MRI scans to examine developmental changes in amygdala (AMY), hippocampus (HIPPO), prefrontal cortex (PFC), as well as functional connectivity (FC) between those regions. Physical growth and pubertal measures increased from pre- to peri-puberty. The indexes of pubertal maturation -testicular size and T- were correlated at peri-puberty, but not at pre-puberty (23 months). Our findings also showed ICV, AMY, HIPPO and total PFC volumetric growth, but with region-specific changes in PFC. Surprisingly, FC in these neural circuits only showed developmental changes from pre- to peri-puberty for HIPPO-orbitofrontal FC. Finally, testicular size was a better predictor of brain structural maturation than T levels -suggesting gonadal hormones-independent mechanisms-, whereas T was a strong predictor of functional connectivity development. We expect that these neural circuits will show more drastic pubertal-dependent maturation, including stronger associations with pubertal measures later, during and after male puberty.

Validation of the Social Responsiveness Scale (SRS) to screen for atypical social behaviors in juvenile macaques.

Primates form strong social bonds and depend on social relationships and networks that provide shared resources and protection critical for survival. Social deficits such as those present in autism spectrum disorder (ASD) and other psychiatric disorders hinder the individual's functioning in communities. Given that early diagnosis and intervention can improve outcomes and trajectories of ASD, there is a great need for tools to identify early markers for screening/diagnosis, and for translational animal models to uncover biological mechanisms and develop treatments. One of the most widely used screening tools for ASD in children is the Social Responsiveness Scale (SRS), a quantitative measure used to identify individuals with atypical social behaviors. The SRS has been adapted for use in adult rhesus monkeys (Macaca mulatta)-a species very close to humans in terms of social behavior, brain anatomy/connectivity and development-but has not yet been validated or adapted for a necessary downward extension to younger ages matching those for ASD diagnosis in children. The goal of the present study was to adapt and validate the adult macaque SRS (mSRS) in juvenile macaques with age equivalent to mid-childhood in humans. Expert primate coders modified the mSRS to adapt it to rate atypical social behaviors in juvenile macaques living in complex social groups at the Yerkes National Primate Research Center. Construct and face validity of this juvenile mSRS (jmSRS) was determined based on well-established and operationalized measures of social and non-social behaviors in this species using traditional behavioral observations. We found that the jmSRS identifies variability in social responsiveness of juvenile rhesus monkeys and shows strong construct/predictive validity, as well as sensitivity to detect atypical social behaviors in young male and female macaques across social status. Thus, the jmSRS provides a promising tool for translational research on macaque models of children social disorders.

Structural development of cortical lobes during the first 6 months of life in infant macaques.

This study mapped the developmental trajectories of cortical regions in comparison to overall brain growth in typically developing, socially-housed infant macaques. Volumetric changes of cortical brain regions were examined longitudinally between 2-24 weeks of age (equivalent to the first 2 years in humans) in 21 male rhesus macaques. Growth of the prefrontal, frontal, parietal, occipital, and temporal cortices (visual and auditory) was examined using MRI and age-specific infant macaque brain atlases developed by our group. Results indicate that cortical volumetric development follows a cubic growth curve, but maturational timelines and growth rates are region-specific. Total intracranial volume (ICV) increased significantly during the first 5 months of life, leveling off thereafter. Prefrontal and temporal visual cortices showed fast volume increases during the first 16 weeks, followed by a plateau, and significant growth again between 20-24 weeks. Volume of the frontal and temporal auditory cortices increased substantially between 2-24 weeks. The parietal cortex showed a significant volume increase during the first 4 months, whereas the volume of the occipital lobe increased between 2-12 weeks and plateaued thereafter. These developmental trajectories show similarities to cortical growth in human infants, providing foundational information necessary to build nonhuman primate (NHP) models of human neurodevelopmental disorders.

Early Developmental Trajectories of Functional Connectivity Along the Visual Pathways in Rhesus Monkeys.

Early social interactions shape the development of social behavior, although the critical periods or the underlying neurodevelopmental processes are not completely understood. Here, we studied the developmental changes in neural pathways underlying visual social engagement in the translational rhesus monkey model. Changes in functional connectivity (FC) along the ventral object and motion pathways and the dorsal attention/visuo-spatial pathways were studied longitudinally using resting-state functional MRI in infant rhesus monkeys, from birth through early weaning (3 months), given the socioemotional changes experienced during this period. Our results revealed that (1) maturation along the visual pathways proceeds in a caudo-rostral progression with primary visual areas (V1-V3) showing strong FC as early as 2 weeks of age, whereas higher-order visual and attentional areas (e.g., MT-AST, LIP-FEF) show weak FC; (2) functional changes were pathway-specific (e.g., robust FC increases detected in the most anterior aspect of the object pathway (TE-AMY), but FC remained weak in the other pathways (e.g., AST-AMY)); (3) FC matures similarly in both right and left hemispheres. Our findings suggest that visual pathways in infant macaques undergo selective remodeling during the first 3 months of life, likely regulated by early social interactions and supporting the transition to independence from the mother.

Effects of Selective Neonatal Amygdala Damage on Concurrent Discrimination Learning and Reinforcer Devaluation in Monkeys.

OBJECTIVES: The amygdala is known to be a key neural structure in many neuropsychiatric disorders. Primarily known for its involvement in fear regulation, the amygdala also plays a critical role in appetitive flexible decision-making. Yet, its contribution to the development of flexible goal-directed behavior has not been thoroughly examined. DESIGN: The current study examined flexible decision-making abilities after neonatal amygdala lesions in nonhuman primates using a behavioral paradigm known to measure the flexible monitoring of goal-directed choices in rodents, monkeys, and humans. METHOD: Rhesus monkeys of both sexes were divided into two groups, a sham-operated control group (N=4) and a group with neonatal neurotoxic amygdala lesions (N=5). Animals received the lesions at 1-2 weeks and were tested at both four and six years of age on a concurrent discrimination reinforcer devaluation task. RESULTS: Although neonatal amygdala damage spared learning stimulus-reward associations, it severely impaired the ability to flexibly shift object choices away from those items associated with devalued food rewards. The results were similar to those obtained in monkeys that had acquired the same lesions in adulthood. CONCLUSIONS: Thus, the amygdala is critical for appetitive decision-making, and provide further evidence of little functional sparing after early amygdala insult. The findings are discussed in relation to other behavioral measures on the same animals and to clinical neuropsychiatric disorders.

Change in background context disrupts performance on visual paired comparison following hippocampal damage.

The medial temporal lobe plays a critical role in recognition memory but, within the medial temporal lobe, the precise neural structures underlying recognition memory remain equivocal. In this study, visual paired comparison (VPC) was used to investigate recognition memory in a human patient (YR), who had a discrete lesion of the hippocampus, and a group of monkeys with neonatal hippocampal lesions, which included the dentate gyrus, and a portion of parahippocampal region. Participants were required to view a picture of an object on a coloured background. Immediately afterwards, this familiar object was shown again, this time paired with a novel object. All participants displayed a novelty preference, provided the background on which the objects were shown was the same as the one used during the learning phase. When the background of the familiar object was changed between initial familiarization and test, only the control subjects showed a novelty preference; the hippocampal-lesioned monkeys and patient YR showed null preference. The results are interpreted within Eichenbaum and Bunsey's [Eichenbaum, H., & Bunsey, M. (1995). On the binding of associations in memory: Clues from studies on the role of the hippocampal region in paired-associate learning. Current Directions in Psychological Science, 4, 19-23] proposal that the hippocampus facilitates the formation of a flexible representation of the elements that make up a stimulus whereas the parahippocampal region is involved in the formation of a fused representation.

Measuring reward assessment in a semi-naturalistic context: the effects of selective amygdala, orbital frontal or hippocampal lesions.

Studying the neural mechanisms underlying complex goal-directed behaviors, such as social behavior, reward seeking or punishment avoidance, has become increasingly tractable in humans, nonhuman primates and rodents. In most experiments, however, goal-directed behaviors are measured in a laboratory setting, which is vastly different from the context in which these behaviors naturally occur. This study adapted a reward assessment paradigm, previously conducted with rhesus monkeys (Macaca mulatta) in the controlled environment of a Wisconsin General Testing Apparatus (WGTA) [Machado CJ, Bachevalier J (2007) The effects of selective amygdala, orbital frontal cortex or hippocampal formation lesions on reward assessment in nonhuman primates. Eur J Neurosci 25:2885-2904], to a more naturalistic context. We used this new paradigm to examine the effects of bilateral amygdaloid, hippocampal or orbital frontal cortex lesions on established food and nonfood preferences. Behavioral modification following reinforcer devaluation was also measured. Consistent with our previous study, none of the lesions produced changes in preference for palatable foods relative to pre-surgery, but animals with amygdala lesions displayed heightened preference for unpalatable foods that control or other operated animals typically avoided. In contrast to several previous WGTA-based experiments, nonfood preference was not affected by any of the lesions. Finally, animals with orbital frontal cortex lesions continued to select preferred foods after satiation, but those with amygdala, hippocampal or sham lesions altered their foraging behavior appropriately and selected less of the sated food. These findings parallel food devaluation results obtained with these same animals when tested in the WGTA. Overall, this study stresses the importance of testing context when measuring decision-making abilities in nonhuman primates with selective brain lesions.

Corpus callosum diffusion anisotropy correlates with neuropsychological outcomes in twins disconcordant for traumatic brain injury.

Conventional and diffusion tensor MR imaging studies in twins sustaining severe pediatric traumatic brain injury identified reduction in fractional anisotropy (FA) in all regions of the corpus callosum, particularly the posterior body, rostral body, and genu, relative to healthy cotwins. FA from the rostrum, genu, anterior body, posterior body, and isthmus were correlated with measures of reading speed and comprehension; verbal working memory and math fact retrieval scores were correlated only with the rostral body FA.

An fMRI study of executive functioning after severe diffuse TBI.

PRIMARY OBJECTIVE: Preliminary study of whether severe diffuse traumatic brain injury (TBI) increases extent of frontal tissue recruited by cognitive control tasks. RESEARCH DESIGN: Functional magnetic resonance imaging (fMRI) on N-back working memory (WM)and arrows inhibition tasks in a 46 year old man who had severe diffuse TBI 1 year earlier, a 44 year old man (inhibition task) and three women (working memory task), age 20-26 years. Images were acquired by 1.5 T magnet with BOLD method and PRESTO pulse sequence and analysed using SPM. MAIN OUTCOMES AND RESULTS: Frontal activation increased under 2-back relative to 1-back condition of working memory in all participants with more extensive activation in the TBI patient relative to controls. Frontal activation increased with inhibition on the arrows task, but was greater in the TBI patient. CONCLUSION: Severe diffuse TBI results in recruitment of additional neural resources for cognitive control.

Neurotoxic lesions of perirhinal cortex impair visual recognition memory in rhesus monkeys.

Recent excitotoxic lesion studies in monkeys have shown that the recognition memory deficits originally attributed to amygdalo-hippocampal damage were due in whole or in part to the accompanying damage to surrounding tissue, including fibers of passage. Here we show that the same conclusion does not apply to the visual recognition impairment produced by aspiration lesions of perirhinal cortex inasmuch as equally severe impairment was found after excitotoxic lesions of this cortex. The finding demonstrates that damage limited to perirhinal neurons is sufficient to impair visual memory and that damage to fibers of passage neither caused nor exacerbated the effect described initially.

Effects of selective neonatal temporal lobe lesions on socioemotional behavior in infant rhesus monkeys (Macaca mulatta).

Normal infant monkeys and infant monkeys with neonatal damage to either the medial temporal lobe or the inferior temporal visual area were assessed in dyadic social interactions at 2 and 6 months of age. Unlike the normal infant monkeys, which developed strong affiliative bonds and little or no behavioral disturbances, the lesioned monkeys (each of which was observed with an unoperated control) exhibited socioemotional abnormalities and aberrant behaviors. The socioemotional changes predominated at 6 months of age and were particularly severe in monkeys with medial temporal lesions. In both the pattern and time course, the socioemotional deficits produced by the neonatal medial temporal lesions bear a striking resemblance to the behavioral syndrome in children with autism. Further analysis of these lesion-induced abnormalities in nonhuman primates may therefore provide insight into this debilitating human developmental disorder.

Volume of focal brain lesions and hippocampal formation in relation to memory function after closed head injury in children.

OBJECTIVES: (1) A study of verbal learning and memory in children who had sustained a closed head injury (CHI) at least 3 months earlier. (2) To relate memory function to focal brain lesion and hippocampal formation volumes using morphometric analysis of MRI. METHODS: A group of 245 children who had been admitted to hospital for CHI graded by the Glasgow coma scale (GCS), including 161 patients with severe and 84 with mild CHI completed the California verbal learning test (CVLT) and underwent MRI which was analysed for focal brain lesion volume independently of memory test data. Brain MRI with 1.5 mm coronal slices obtained in subsets of 25 patients with severe and 25 patients with mild CHI were analysed for hippocampal formation volume. Interoperator reliability in morphometry was satisfactory. RESULTS: Severity of CHI and age at study significantly affected memory performance. Regression analysis showed that bifrontal, left frontal, and right frontal lesion volumes incremented prediction of various learning and memory indices after entering the GCS score and age into the model. Extrafrontal lesion volume did not contribute to predicting memory performance. CONCLUSIONS: Prefrontal lesions contribute to residual impairment of learning and memory after severe CHI in children. Although effects of CHI on hippocampal formation volume might be difficult to demonstrate in non-fatal paediatric CHI, further investigation using functional brain imaging could potentially demonstrate hippocampal dysfunction.

Effects of neonatal inferior prefrontal and medial temporal lesions on learning the rule for delayed nonmatching-to-sample.

The ability of rhesus monkeys to master the rule for delayed nonmatching-to-sample (DNMS) has a protracted ontogenetic development, reaching adult levels of proficiency around 4 to 5 years of age (Bachevalier, 1990). To test the possibility that this slow development could be due, at least in part, to immaturity of the prefrontal component of a temporo-prefrontal circuit important for DNMS rule learning (Kowalska, Bachevalier, & Mishkin, 1991; Weinstein, Saunders, & Mishkin, 1988), monkeys with neonatal lesions of the inferior prefrontal convexity were compared on DNMS with both normal controls and animals given neonatal lesions of the medial temporal lobe. Consistent with our previous results (Bachevalier & Mishkin, 1994; Málková, Mishkin, & Bachevalier, 1995), the neonatal medial temporal lesions led to marked impairment in rule learning (as well as in recognition memory with long delays and list lengths) at both 3 months and 2 years of age. By contrast, the neonatal inferior convexity lesions yielded no impairment in rule-learning at 3 months and only a mild impairment at 2 years, a finding that also contrasts sharply with the marked effects of the same lesion made in adulthood. This pattern of sparing closely resembles the one found earlier after neonatal lesions to the cortical visual area TE (Bachevalier & Mishkin, 1994; Málková et al., 1995). The functional sparing at 3 months probably reflects the fact that the temporo-prefrontal circuit is nonfunctional at this early age, resulting in a total dependency on medial temporal contributions to rule learning. With further development, however, this circuit begins to provide a supplementary route for learning.

Effects of aspiration versus neurotoxic lesions of the amygdala on emotional responses in monkeys.

All previous reports describing alterations in emotional reactivity after amygdala damage in monkeys were based on aspiration or radiofrequency lesions which likely disrupted fibres of passage coursing to and from adjacent ventral and medial temporal cortical areas. To determine whether this associated indirect damage was responsible for some or all of the changes described earlier, we compared the changes induced by aspiration of the amygdala with those induced by fibre-sparing neurotoxic lesions. Four different stimuli, two with and two without a social component, were used to evaluate the expression of defence, aggression, submission and approach responses. In unoperated controls, defence and approach behaviours were elicited by all four stimuli, 'social' and inanimate alike, whereas aggression and submission responses occurred only in the presence of the two 'social' stimuli. Furthermore, all defence reactions were reduced with an attractive inanimate item, while freezing was selectively increased with an aversive one. Relative to controls, monkeys with neurotoxic amygdala lesions showed the same array of behavioural changes as those with aspiration lesions, i.e. reduced fear and aggression, increased submission, and excessive manual and oral exploration. Even partial neurotoxic lesions involving less than two-thirds of the amygdala significantly altered fear and manual exploration. These findings convincingly demonstrate that the amygdala is crucial for the normal regulation of emotions in monkeys. Nevertheless, because some of the symptoms observed after neurotoxic lesions were less marked than those seen after aspiration lesions, the emotional disorders described earlier after amygdalectomy in monkeys were likely exacerbated by the attendant fibre damage.

Memory and socioemotional behavior in monkeys after hippocampal damage incurred in infancy or in adulthood.

The present study reviews the long-term effects of neonatal hippocampal damage in monkeys on the development of memory functions and socioemotional behavior. The results showed that neonatal damage to the hippocampal formation impairs specific memory processes, such as those subserving automatic (as opposed to effortful) recognition memory and relational learning, while sparing the abilities to acquire skills, such as object discriminations. Furthermore, the neonatal hippocampectomy led to a progressive loss of social affiliation and a protracted emergence of locomotor stereotypies. While the memory losses following neonatal hippocampal lesions resemble those found after similar lesions acquired in adulthood, only the neonatal lesions resulted in a protracted emergence of abnormal behaviors. These later findings suggested that, presumably, the neonatal lesions impacted on neural systems remote from the site of damage. This was confirmed by our more recent neurobiological studies, demonstrating that neonatal, but not late, lesions of the medial temporal lobe region, disrupt the normal behavioral and cognitive processes subserved by the prefrontal cortex and the caudate nucleus. All together the data support the neurodevelopmental hypothesis viewing early insult to the medial temporal region as the origin of developmental psychosis in humans, such as schizophrenia.

Striatal dopamine receptors and transporters in monkeys with neonatal temporal limbic damage.

Developmental cortical damage has been implicated in the basic neurobiology of schizophrenia. Adult rhesus monkeys with neonatal temporal limbic damage show a stimulus-dependent disinhibition of subcortical dopamine (DA) release. We measured dopamine D2 receptors and transporters in vivo in rhesus monkeys with neonatal and adult mesial temporal limbic lesions and control monkeys to explore further the effects of this developmental lesion on striatal DA function. All monkeys were studied with [I-123]IBZM SPECT to assess the availability of striatal dopamine D2 receptors and with [I-123]beta-CIT SPECT to measure the availability of dopamine transporters in the striatum. IBZM binding was significantly reduced in monkeys with neonatal limbic lesions. No group difference in beta-CIT binding was found. The reduction in IBZM binding was significantly correlated with subcortical dopamine release after monoaminergic prefrontal stimulation as determined with in vivo microdialysis. Our findings imply specific interactions between age at lesion and the availability of DA transporter and receptors in non-human primates, and suggest that stimulus-dependent DA activity affects the expression of DA receptors.

The relationship between dorsolateral prefrontal N-acetylaspartate measures and striatal dopamine activity in schizophrenia.

BACKGROUND: Pathology of dorsolateral prefrontal cortex and dysregulation of dopaminergic neurons have been associated with the pathophysiology of schizophrenia, but how these phenomena relate to each other in patients has not been known. It has been hypothesized that prefrontal cortical pathology might induce both diminished steady-state and exaggerated responses of dopaminergic neurons to certain stimuli (e.g., stress). We examined the relationship between a measure of prefrontal neuronal pathology and striatal dopamine activity in patients with schizophrenia and in a nonhuman primate model of abnormal prefrontal cortical development. METHODS: In the patients, we studied in vivo markers of cortical neuronal pathology with NMR spectroscopic imaging and of steady-state striatal dopamine activity with radioreceptor imaging. In the monkeys, we used the same NMR technique and in vivo microdialysis. RESULTS: Measures of N-acetyl-aspartate concentrations (NAA) in dorsolateral prefrontal cortex strongly and selectively predicted D2 receptor availability in the striatum (n = 14, rho = -.64, p < .01), suggesting that the greater the apparent dorsolateral prefrontal cortex pathology, the less the steady-state dopamine activity in these patients. A similar relationship between NAA measures in dorsolateral prefrontal cortex and steady-state dopamine concentrations in the striatum was found in the monkeys (n = 5, rho = .70, p < .05). We then tested in the same monkeys the relationship of prefrontal NAA and striatal dopamine overflow following amphetamine infusion into dorsolateral prefrontal cortex. Under these conditions, the relationship was inverted, i.e., the greater the apparent dorsolateral prefrontal cortex pathology, the greater the dopamine release. CONCLUSIONS: These data demonstrate direct relationships between putative neuronal pathology in dorsolateral prefrontal cortex and striatal dopamine activity in human and nonhuman primates and implicate a mechanism for dopamine dysregulation in schizophrenia.

Long-term effects of neonatal damage to the hippocampal formation and amygdaloid complex on object discrimination and object recognition in rhesus monkeys (Macaca mulatta).

Rhesus monkeys with neonatal aspiration lesions of the hippocampal formation or the amygdaloid complex were tested on concurrent discrimination learning (24-hr intertrial interval [ITI]) at 3 months, on object recognition memory (delayed nonmatching-to-sample [DNMS]) at 10 months, and retested on both tasks at 6-7 years of age. Neonatal amygdaloid damage mildly impaired acquisition at the 24-hr ITI and the performance test of DNMS at both ages. In contrast, early hippocampal lesions impaired performance only on the longest lists of 10 items in DNMS in adult monkeys. Thus, early amygdala lesions appeared to have resulted in a greater object memory loss than early hippocampal lesions. However, in light of recent findings from lesion studies in adult monkeys, the object memory impairment after early amygdaloid lesions is better accounted for by damage to the entorhinal and perirhinal cortex than by damage to the amygdaloid nuclei.

Neonatal aspiration lesions of the hippocampal formation impair visual recognition memory when assessed by paired-comparison task but not by delayed nonmatching-to-sample task.

Previous experiments showed that neonatal aspiration lesions of the hippocampal formation in monkeys yield no visual recognition loss at delays up to 10 min, when recognition memory was assessed by a trial-unique delayed nonmatching-to-sample (DNMS) task. The present study examined whether neonatal hippocampal lesions also have no effect on visual recognition when assessed by a visual paired-comparison (VPC) task. In the VPC task, animals are looking at visual stimuli and their preference for viewing new stimuli is measured. Normal adult monkeys showed strong preference for looking at the novel stimuli at all delays tested. By contrast, adult monkeys with neonatal hippocampal lesions, which included the dentate gyrus, cornus ammon (CA) fields, subicular complex, and portions of parahippocampal areas TH/TF, showed preference for novelty at short delays of 10 s but not at longer delays of 30 s to 24 h. This visual recognition loss contrasts with the normal performance of the same operated animals when tested in the DNMS task. The discrepancy between the results obtained in the two recognition tasks suggests that, to perform normally on the DNMS task, the operated monkeys may have used behavioral strategies that do not depend on the integrity of the hippocampal formation. In this respect, VPC appears to be a more sensitive task than DNMS to detect damage to the hippocampal region in primates.